onAttach method
Handles any setup of the renderer that needs to be deferred until it is attached to a chart.
Implementation
@override
void onAttach(BaseChart<D> chart) {
super.onAttach(chart);
// We only need the chart.context.isRtl setting, but context is not yet
// available when the default renderer is attached to the chart on chart
// creation time, since chart onInit is called after the chart is created.
_chart = chart;
}